This technology relates to a method and system of using solar energy to drive the thermally activated electrosynthesis of energetic molecules.
One third of the global industrial sector's annual emission of 1×1010 metric tons of the greenhouse gas, CO2, is released in the production of metals and chlorine. This, together with the additional CO2 emissions for electrical generation, heating and transportation, comprise the majority of anthropogenic CO2 emissions.
Photoelectrochemical solar cells (PECs) can convert solar energy to electricity and with inclusion of an electrochemical storage couple, have the capability for internal energy storage, to provide a level output despite variations in sunlight. Solar energy can also be stored externally in chemical form, when it is used to drive the formation of energetically rich chemicals. As an example in 2001, external multiple bandgap PVs (photovoltaics) were used to generate H2 by splitting water at 18% solar energy conversion efficiency. In 2002, a hybrid photo-thermal electrochemical theory was introduced, and verified by experiment in 2003, for H2 generation at over 30% solar energy conversion efficiency.
Light driven water splitting was originally demonstrated with TiO2 (a semiconductor with a bandgap, Eg, >3.0 eV). However, only a small fraction of sunlight has sufficient energy to drive TiO2 photoexcitation, and studies had sought to tune (lower) the semiconductor bandgap to provide a better match to the electrolysis potential. An alternative approach is to tune (lower) the electrolysis potential, as was demonstrated with Si (Eg=1.1 eV) solar driven, high temperature water electrolysis. With increasing temperature, the quantitative decrease in the electrochemical potential to split water to hydrogen and oxygen had been well known by the 1950's, and as early as 1980 it was noted that solar thermal energy could decrease the necessary energy for the generation of H2 by electrolysis. However, the process combines elements of solid state physics, insolation and electrochemical theory, complicating rigorous theoretical support of the process. The first hybrid photo-thermal electrochemical theory for the solar generation of H2 was developed in 2002. The thermal/electrochemical hybrid model for solar/H2 by this process, was the first derivation of bandgap restricted, thermal enhanced, solar water splitting efficiencies. The model was initially derived for AM1.5 (terrestrial insolation), and later expanded to include sunlight above the atmosphere (AM0 insolation). The experimental accomplishment of 30% solar H2 conversion efficiency followed, establishing that the water splitting potential can be specifically tuned to match efficient photo-absorbers, eliminating the challenge of tuning (varying) the semiconductor bandgap, and can lead to over 30-50% solar energy conversion to H2 efficiencies. That process was specific to H2 and does not contemplate the production of other energetically rich chemicals.